On a practical level, corporate and organizational “personhood” has worked coherently for more than a century. Will this? [Reuters, Science; earlier on corporate personhood ("established and relatively uncontroversial," and progressive in its legal implications)] A Twitter reaction: “If they get the right to air political ads they can only improve the discourse.” [@jacobgrier]
More seriously, Prof. Bainbridge provided an answer to the question both on Twitter (“We treat corporations as people because it is a useful fiction. Animals as persons is not useful.”) and then in a longer blog post, which concludes:
The problem, I believe, is that attempts to define the debate in moral or philosophical terms ignores the basic fact that the rationale for corporate personhood sounds in neither. Instead, it is based on practicality and utility. Put another way, we treat the corporation as a legal person because doing so has proven to be a highly efficient way for real people to organize their business activities and to vindicate their rights. Put yet another way, we treat the corporation as a legal person because it is a nexus of contracts between real persons. Which is something no animal can ever be.